The Irish Independent has learned that six local authorities have been testing drinking water after urea was spread on national roads in order to melt ice during the big freeze.

It is feared that as the ice and snow melted, the urea may have run off into water sources, some of which could supply drinking water to households and businesses.

Urea, a naturally occurring substance found in urine but which is produced commercially, is 46pc nitrogen and excess use can burn vegetation and contaminate water bodies.

But its high-nitrate content means it is a natural de-icer and it is commonly used in airports to keep runways clear. It works the same way as rock salt, by melting ice and snow.

Alarms triggered

Some 210 tonnes were spread in Kildare, Carlow, Meath, South Dublin, North Tipperary and South Tipperary after stocks of rock salt had fallen to critically low levels.

Last night, it emerged that alarms were triggered at the Leixlip, Co Kildare treatment plant last weekend when high levels of nitrates showed up in the water supply. The nitrates were subsequently removed through treatment and the supply was then passed as safe.

But on Tuesday, the six local authorities were contacted and asked to test drinking water in their areas in order to determine whether or not their supplies had been contaminated.

The urea was used after the National Emergency Response Co-Ordination Committee (NERCC) asked the National Roads Authority (NRA) to source 600 tonnes of urea for use as a de-icer on an emergency basis.

The request came after Met Eireann had predicted sub-zero temperatures and snowfall across the east of the country last weekend.

A Government spokesman said: "With all the available information on Friday, the decision was either to use this, particularly in Kildare, or close motorways. Now, the issues arising from it will be dealt with and safety will be the paramount issue."

The NRA confirmed that it had sourced 600 tonnes of the product, of which 210 tonnes were used. Usage within individual counties ranged between 12 and 80 tonnes.